The present invention relates to steam turbine control and performance and, more particularly, to a control algorithm and system effecting loading and unloading of steam turbines while maintaining life cycle stress within limits by the use of controlled heating of the turbine shell and rotor.
Power generation steam turbines have large diameter rotors and thick shells. The rotors experience stress as the result of centrifugal loading and thermal expansion during loading and unloading cycles. During each start/stop cycle, the rotor and shell accumulate low cycle fatigue. When the low cycle fatigue accumulated exceeds the material limits, there exists a probability of crack formation, and the equipment must be replaced. The level of low cycle fatigue damage accumulated during each start cycle is a function of the peak stress experience for that start cycle. The turbine inlet steam temperature and flow determine the rate of metal temperature change and thus thermal stress in the shell and rotor.
The current method to control thermal stress in steam turbines involves estimating rotor stress as a function of a temperature measurement, typically shell metal near the turbine inlet. As the stress estimate approaches the low cycle fatigue limit, the opening rate of the steam turbine inlet valve is reduced. The disadvantage to this approach is that peak stress occurs ten to fifteen minutes after thermal transients. Because the time delay is so long, in order to prevent high stress cycles, very low steam flow rates are required. This limits the operability of the turbine and extends startup time.
It would thus be desirable to effect control of steam flow to the steam turbine in such a way as to limit thermal stress to an acceptable level while minimizing start times and maximizing operability.